Metal-air cell with performance enhancing additive

ABSTRACT

Systems and methods drawn to an electrochemical cell comprising a low temperature ionic liquid comprising positive ions and negative ions and a performance enhancing additive added to the low temperature ionic liquid. The additive dissolves in the ionic liquid to form cations, which are coordinated with one or more negative ions forming ion complexes. The electrochemical cell also includes an air electrode configured to absorb and reduce oxygen. The ion complexes improve oxygen reduction thermodynamics and/or kinetics relative to the ionic liquid without the additive.

RELATED APPLICATION

This application claims priority from U.S. Provisional Application Ser.No. 61/334,047, filed May 12, 2010, which is hereby incorporated byreference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under ARPA-e grant No.AR0000038, awarded by the Advanced Research Projects Agency of theDepartment of Energy. The United States government has certain rights inthis invention.

All publications, patents, and patent applications cited in thisSpecification are hereby incorporated by reference in their entirety.

BACKGROUND

Metal-air batteries typically include a fuel electrode at which metalfuel is oxidized, an air electrode at which oxygen is reduced, and anelectrolyte for providing ion conductivity. A significant limitingfactor with conventional metal-air batteries is the evaporation of theelectrolyte solution, particularly the evaporation of the solvent, suchas water in an aqueous electrolyte solution. Because the air electrodeis required to be air permeable to absorb oxygen, it also may permit thesolvent vapor, such as water vapor, to escape from the cell. Over time,the cell becomes incapable of operating effectively because of thedepletion of the electrolyte. Indeed, in many cell designs thisevaporation issue renders the cell inoperable before the fuel isconsumed. And this issue is exacerbated in secondary (i.e.,rechargeable) cells, because the fuel may be re-charged repeatedly overthe life of the cell, whereas the electrolyte solution cannot (absentreplenishment from an external source). Also, in rechargeable cells, thewater solvent is typically oxidized to evolve oxygen during re-charge,which also depletes the solution.

To compensate for this problem, metal-air batteries with aqueouselectrolyte solutions are typically designed to contain a relativelyhigh volume of electrolyte solution. Some cell designs even incorporatemeans for replenishing the electrolyte from an adjacent reservoir tomaintain the electrolyte level. However, either approach adds to boththe overall size of the cell, as well as the weight of the cell, withoutenhancing the cell performance (except to ensure that there is asignificant volume of electrolyte solution to offset evaporation of thewater or other solvent over time). Specifically, the cell performance isgenerally determined by the fuel characteristics, the electrodecharacteristics, the electrolyte characteristics, and the amount ofelectrode surface area available for reactions to take place. But thevolume of electrolyte solution in the cell generally does not have asignificant beneficial effect on cell performance, and thus generallyonly detracts from cell performance in terms of volumetric and weightbased ratios (power to volume or weight, and energy to volume orweight). Also, an excessive volume of electrolyte solution may create ahigher amount of spacing between the electrodes, which may increaseohmic resistance and detract from performance.

SUMMARY

Embodiments provided herein are related to electrochemical metal-aircells and more particularly to an electrochemical metal-air cell havingan ionically conductive medium, which can include an additive thatimproves oxygen reduction thermodynamics, kinetics, or both.

An embodiment is related to an electrochemical cell comprising: a fuelelectrode for oxidizing a metal fuel; a low temperature ionic liquidcomprising positive ions and negative ions; an oxygen reductionenhancing compound added to the low temperature ionic liquid to formoxygen reduction enhancing positive ions, wherein the oxygen reductionenhancing positive ions are coordinated with one or more negative ionsforming oxygen reduction enhancing positive-negative ion complexes; andan air electrode configured to absorb and reduce oxygen, wherein theoxygen reduction enhancing positive-negative ion complex improves oxygenreduction thermodynamics, kinetics, or both, relative to the ionicliquid without the oxygen reduction enhancing compound.

An alternative embodiment is related to a method comprising: mixing anoxygen reduction enhancing compound with a low temperature ionic liquidto create a solution comprising an oxygen reduction enhancingpositive-negative ion complex; exposing the solution to oxygen; andelectrochemically reducing the oxygen.

Another embodiment is related to an electrochemical cell comprising: ametal fuel electrode for oxidizing a metal fuel; a low temperature ionicliquid comprising positive ions and negative ions; a local oxideformation promoting compound added to the low temperature ionic liquid,the local oxide formation promoting additive dissolving in the lowtemperature ionic liquid to form local oxide formation promotingpositive ions, wherein the positive ions of the local oxide formationpromoting compound are coordinated with one or more negative ionsforming local oxide formation promoting positive ion-negative ioncomplexes; and an air electrode configured to absorb and reduce oxygen,and store oxides of the metal fuel; wherein the local oxide formationpromoting positive ion-negative ion complex increases the formation andstorage of oxides of the metal fuel at the air electrode duringdischarge relative to the ionic liquid without the local oxide formationpromoting compound.

Another embodiment is related to an electrochemical cell comprising: afuel electrode for oxidizing a metal fuel; an ionically conductivemedium comprising at least one aprotic ionic liquid and at least oneprotic ionic liquid comprising at least one available proton per ionpair; and an air electrode configured to absorb and reduce oxygen.

An alternative embodiment is related to a method comprising: mixing aprotic ionic liquid with an aprotic ionic liquid to create an ionicallyconductive medium comprising negative ions and positive ions, wherein atleast one of the positive ions is a proton; exposing the ionicallyconductive medium to oxygen; and electrochemically reducing the oxygen.

Another embodiment is related to an electrochemical cell comprising: ametal fuel electrode for oxidizing a metal fuel; an ionically conductivemedium comprising at least one aprotic ionic liquid and at least oneprotic ionic liquid comprising at least one proton; an air electrodeconfigured to absorb and reduce oxygen and store oxides of the metalfuel, wherein the proton increases the formation and storage of oxidesof the metal fuel at the air electrode during discharge relative to theionically conductive medium without the protic ionic liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cyclic voltammogram illustrating improvement in kinetics andthermodynamics of a metal ion-ionic liquid solution according to anembodiment.

FIG. 2 illustrates cyclic voltammograms of various embodiments including(a) 0.01M Mn(II) acetate+triethylammonium methansulfonate, (b) 1MZnCl₂+diethylmethylammonium triflate, (c) 5MAlCl₃+1-butyl-3-methylimidazolium bis(trifluoromethane)sulfonamide, and(d) GaCl₃+1-methyl-3-octylimidazolium tetrachlorogallate (1:1).

FIG. 3 is a schematic diagram of an electrochemical cell according to anembodiment.

FIG. 4 is a flow diagram illustrating a method embodiment.

FIG. 5( a) shows a photograph of an experimental setup in oneembodiment; FIG. 5( b) shows a cartoon of a close-up of the measurementtube/chamber; FIGS. 5( c)-(d) respectively show a cyclic voltammograms(CV) and the result of the Koutecky-Levich (“L-K”) analysis with respectto the number of electrons per O₂ vs. potential; FIGS. 5( e)-(f) showadditional background ORR analysis data regarding linear sweepvoltammetry (LSV) at 10 mV/s in N2 deaerated with 62 ppm water forBDMelm:Tf and some properties thereof, respectively.

FIG. 6 shows protic additives and their associated pKa's as observed inone embodiment.

FIGS. 7( a)-(b) show cyclic voltammograms of the embodiment with wateras an additive on Pt (7(a)) and glassy carbon (“GC”) disk (7(b)) at 50°C.; FIGS. 7( c)-(d) show additional data regarding LSV at 10 mV/s onboth Pt disk (7(c)) and GC disk (7(d)) at 50° C.; FIGS. 7( e)-(f) showthe results of Koutecky-Levich (“L-K”) analysis with respect to thenumber of electrons per O₂ vs. potential for Pt disk (7(e)) and GC disk(7(f)).

FIGS. 8( a)-(c) show cyclic voltammograms of the embodiment with HTf asthe protic additive; FIGS. 8( d)-(e) show the results of the K-Kanalysis.

FIGS. 9( a)-(b) show cyclic voltammograms of the embodiment with HMeS asthe protic additive; FIGS. 9( c)-(d) show the results of the L-Kanalysis.

FIGS. 10( a)-(b) show cyclic voltammograms of the embodiment with PryH⁺as the protic additive; FIGS. 10( c)-(d) show the results of the L-Kanalysis.

FIGS. 11( a)-(b) show the results of the L-K analysis in one embodimentwith TFEtOH as the protic additive.

FIG. 12 shows the solvent leveling effects as observed in the oxygenreduction reaction onsets versus pKa of the protic additive in water forboth Pt and GC disks.

FIGS. 13( a)-(b) show the oxygen reduction reaction turn-on potential asa function of pKa for Pt and GC.

DETAILED DESCRIPTION

One embodiment described herein is an electrochemical cell comprising afuel electrode, an air electrode, and an ionically conductive medium.The cell can be, for example, part of a metal-air battery, such as arechargeable metal-air battery; the battery can also benon-rechargeable. A fuel electrode can oxidize a fuel, such as a metalfuel in the embodiment of a metal-air battery. The air electrode can beconfigured to absorb and reduce air, such as oxygen, to store theby-products, such as oxides.

Oxygen Reduction Enhancing Compound

The addition of an oxygen reduction enhancing compound to a lowtemperature ionic liquid containing medium of a metal-air battery mayimprove oxygen reduction thermodynamics and/or kinetics relative to thesame medium without the added compound. It is believed that the oxygenreduction enhancing compound can dissociate into oxygen reductionenhancing positive ions (“cations”), which can coordinate with one ormore negative ions (“anions”) to form oxygen reduction enhancingpositive-negative ion complexes.

The oxygen reduction enhancing compound can be a variety of compounds.For example, it can be in the form of an additive. The additive cancomprise an inorganic or an organic molecule. The additive can alsocomprise water. Alternatively, the additive can comprise a metal (as ina metal-containing additive), an organic molecule, water, any of theadditives described below, or combinations thereof.

The additive can be a metal-containing additive. The metal in themetal-containing additive can be any suitable metal. For example, themetal in the metal-containing additive in one embodiment can be at leastone of Mg, Al, Mn, Ga, and Zn. In one embodiment, it is believed thatthe metal containing compound dissociates into positive metal ions,which associate and/or coordinate with negative ions in solution to forma metal centered-negative ion complex, which in turn enhances the oxygenreduction reaction thermodynamics and/or kinetics relative to the ionicliquid without the compound.

The organic molecule can be any suitable organic molecule. For example,it can be a protic organic molecule, thus the additive can be referredto as a protic organic molecule containing additive. The term protic isfurther described below. The organic molecule containing additive cancomprise triflic acid (HTf), benzonitrile: HTf, acetophenone: HTf,methanesulfonic acid, hydronium triflate, pyridazinium triflate, aceticacid, pyridinium triflate, 1,2-dimethylimidaozlium triflate,n,n-diethyl-n-methylammonium triflate, 2,2,2-trifluoroethanol,2-butyl-1,1,3,3-tetramethylguanidinium triflate, or combinationsthereof. Note that triflic acid (HTf) can sometimes be referred to astrifluoromethanesulfonic acid.

The term “complex” and the term “complexation” are generally known inthe art. In one embodiment, a complex can be a compound, a molecule, ora plurality of ions being in close proximity to one another withoutbeing chemically bonded. In one alternative embodiment, some chemicalbonding can also be present. The attractive force that allows the ionsto be in such proximity can arise from sources such as van der waalsforce, hydrogen bonding, and the like. Note that the formation of acomplex is not limited to the scenarios described herein.

The oxygen reduction reaction, and thus the improvement thereof, can beevaluated based on thermodynamics, kinetics, or both, of the reaction.In one embodiment, thermodynamics is a metric that can be used toevaluate a system at equilibrium, while kinetics is one that can be usedto evaluate a system based on a temporal change—e.g., its reaction rate.One example of a thermodynamics parameter is voltage (or electricalpotential)—e.g., turn-on potential, half-wave potential, etc. On theother hand, one example of a kinetics parameter is current (or currentdensity). Accordingly, the improvement in thermodynamics may be measuredas a shift in the turn-on potential for oxygen reduction or a shift inthe half wave potential. The improvement in kinetics may be measured bythe increase in current density at a given potential. Additionally, thepresence of metal ion-negative ion complexes may enhance thereversibility of an air cathode of the metal-air ionic liquid battery.The improvement in reversibility may be particularly beneficial withaprotic ionic liquids, as discussed in more detail below.

The redox reactions in the electrochemical cell described herein caninvolve the transfer of different numbers of electrons. For example, inone embodiment, the oxygen reduction half-reaction can involve thetransfer of at least two electrons per oxygen molecule, such as at leastfour electrons per oxygen molecule; it can also be a combination ofthese two, as discussed below. Each of these reactions can have itsadvantages. For example, an one-electron oxygen reduction reaction canbe highly reversible, efficient, but has low power density, cellpotential, and/or reactivity. By contrast, a four-electron reaction canhave the highest power density but have low round-trip (RT) efficiencyand may need a catalyst (e.g., peroxide). A two-electron reaction canhave intermediate power/efficiency, but at the same time may faceperoxide instability challenges. On the other hand, the cathode for anone-electron reaction is preferably ultrathin, invariant, and/orwater-proof, whereas that for a four-electron reaction is preferablythin, invariant, and/or with bifunctionality. The cathode for atwo-electron reaction generally can be thick, variant, and/or porous. Inone embodiment, a “bifunctional” electrode can be used for both oxygenreduction and oxygen gas evolution, whereas an invariant electrode canonly be used for one of these two functions, but not both.

Ionic Liquids

The ionically conductive medium can comprise at least one ionic liquid(“IL”). Ionic liquids generally refer to salts that form stable liquidscomprising ions. That is, ionic liquids are fully dissociated,consisting essentially of negative and positive ions. Thus, ionicliquids inherently conduct electricity. Further, ionic liquids havenegligible vapor pressure, low viscosity, wide liquidus range (up to400° C.), tunable hydrophobicity, high thermal stability, and a largeelectrochemical window (>5V). Because of these properties, ionic liquidstypically will not evaporate or be consumed during the charge/dischargecycle of an electrochemical cell.

Presently described embodiments include ionic liquids that are lowtemperature IL. The ILs can have a vapor pressure at or below 1 mm Hg at20° C. above its melting point, and preferably at or below 0.1 mmHg orzero or essentially immeasurable at 20° C. above its melting point. Roomtemperature ionic liquids (“RTIL”) are salts which form a stable liquidat 100° C. or below at 1 atm pressure (i.e., they have a melting pointat 100° C. or below at 1 atm). In one embodiment, a low temperatureionic liquid is defined as an ionic liquid having a melting point at orbelow 150° C. at 1 atm. Low temperature ionic liquids may also includethe various RTILs. Some examples of low temperature ILs can includetriethylammonium methanesulfonate, 1-methyl-3-octylimidazoliumtetrachlorogallate, diethylmethylammonium triflate, and1-butyl-3-methylimidazolium bis(trifluoromethane)sulfonamide, orcombinations thereof.

The presently described low temperature ionic liquid can also be onethat comprises an anion selected from the group consisting oftetrachloroaluminate, bis(trifluoromethylsulfonyl)imide,methylsulfonate, nitrate, and acetate, or derivatives and/orcombinations thereof. Alternatively, the presently described lowtemperature ionic liquid can also be one that comprises a cationselected from the group consisting of imidazolium, sulfonium,pyrrolidinium, pyridinium, triethylammonium, diethylmethylammonium,dimethylethylammonium, ethylammonium, α-picolinium,1,8-bis(dimethylamino)naphthalene, 2,6-di-tert-butylpyridine,quaternized ammonium and phosphonium, or derivatives and/or combinationsthereof.

Even though low temperature or room temperature ionic liquids aredefined by their respective melting points at 1 atm, in some embodimentsthe cell may be operated in an environment with a different pressure,and thus the melting point may vary with the operating pressure. Thus,reference to a melting point at 1 atm is used only as a reference pointto define these liquids, and does not imply or restrict its actual useconditions in operation.

ILs can come in two forms: protic and aprotic. Protic ILs have availableprotons which may be oxidized or reduced or may coordinate with negativeions, such as reduced oxygen. In one embodiment, a protic IL can be anionic liquid formed by proton transfer from a Brönsted acid (HA) to aBrönsted base (B), such as

HA+B→A⁻+BH⁺.

When the cation of the protic ionic liquid contains a reversible proton,it is referred to the fact that the reaction above is reversible. Bycontrast, a “strongly bound proton” herein refers to the fact that theproton transfer energetics can disfavor rendering the reaction above notreversible.

In some incidence, an IL can be referred to as protic if it hasreversibly electrochemically available proton(s). Namely, thedeprotonated leaving group of the IL molecule does not result in adecomposition pathway. These available protons of a protic IL canincrease the oxygen reduction reaction. Note that the term protic canalso be used to describe an additive, or any other compound that has theaforedescribed protic property. In one embodiment wherein the ionicliquid comprises a protic ionic liquid, the protic ionic liquid cancomprise at least one cation comprising at least one reversible proton.

Some examples of protic ILs are synthesized from combinations of anionstetrachloroaluminate, bis(trifluoromethylsulfonyl)imide,methylsulfonate, nitrate, and acetate, and cations triethylammonium,diethylmethylammonium, dimethylethylammonium, ethylammonium,α-picolinium, pyridinium, and 1,8-bis(dimethylamino)naphthalene,2,6-di-tert-butylpyridine, and derivatives of the guanadines, orderivatives and/or combinations thereof.

Aprotic ILs generally do not have proton activity. Some examples ofaprotic RTILs are synthesized from combinations of anions chloride (CL),hexafluorophosphate (PF₆ ⁻), iodide, tetrafluoroborate,bis(trifluoromethylsulfonyl)imide (C₂F₆NO₄S₂ ⁻),trifluoromethanesulfonate (CF₃O₃S⁻), and cations imidazolium, sulfonium,pyrrolidinium, quaternized ammonium or phosphonium and theirderivatives. Despite a lack of proton activity, an aprotic IL cancomprise a proton. For example, an aprotic ionic liquid can comprise atleast one cation that has at least one strongly bound proton thereto.Many other options of ILs exist, and these lists of examples are notintended to be limiting in any way.

In some embodiments, ionic liquid is highly hydrophobic. In oneembodiment, the ionically conductive medium is also hydrophobic. Inthese embodiments, the water content of the electrolyte is essentiallyzero, or has water contents below 10 ppm. In another embodiment, asdescribed above, water can be added as an additive to the ionicallyconductive medium in order to improve oxygen reduction thermodynamics,kinetics, or both. Note that the addition of water can render theelectrolyte (or the ionically conductive medium) system an aqueoussystem. Thus, the IL in water can also be referred to as an electrolytein a solution. For example, water contents of about 5 to about 100,000ppm, such as about 10 to about 50,000 ppm, such as about 100 to about10,000 ppm, such as about 500 to about 5,000 ppm, can be added. It hasbeen found by the present inventors that some addition of water canimprove oxygen reduction of aprotic systems.

Tuning the hydrophobicity such that the solubility is in the range of10-50,000 ppm can enable establishment of a constant water activitywithin the IL. In still other embodiments, a protic IL may be added toan aprotic IL. The addition may be performed via titration or any othersuitable method. In this manner, protons can be added to a predominatelyaprotic IL, thereby improving the oxygen reduction reaction. Indeed,because the addition of the protic IL may be precisely controlled, theproton activity may be tailored as desired.

Reference may be made to U.S. patent application Ser. Nos. 61/267,240,61/177,072, 12/776,962, and 13/085,714, each of which is incorporated byreference in its entirety, for further details concerning theconstruction and operation of a metal-air low temperature ionic liquidcell.

Electrochemical Cell

The air electrode can comprise a polymer, such aspolytetrafluoroethylene (PTFE). The air electrode can also comprise acatalyst. The type of catalyst can vary, depending on the chemistry ofthe electrochemical cell. For example, the catalyst can be at least oneof manganese oxide, nickel, pyrolized cobalt, porphyrin-based catalysts,activated carbon, perovskites, spinels, silver, platinum, and/ormixtures thereof. The air electrode can be permeable to air/gas. In oneembodiment, the air electrode can further comprise a barrier membrane onone of its outer surface. The barrier membrane can be impermeable togas, liquid, or both. In some instances, the air electrode can repel theionically conductive medium, including the ionic liquid containedtherein. In one embodiment, for example, the air electrode can repel alow temperature ionic liquid.

The fuel electrode can be porous. In one embodiment, the fuel electrodecan comprise a backing The backing can be, for example, impermeable toliquid, air, or both. Metal-oxide by-products can be formed during theoperation of an electrochemical cell. The by-products can be formed atthe fuel electrode. Further, the by-products can be stored at the fuelelectrode. Similarly, such metal-oxide by-products can be formed and/orstored at the air electrodes.

In a metal-air battery, the metal is the fuel. That is, during dischargethe metal is oxidized at the anode, providing electrons which can beused for electrical work. The oxidation reaction may be represented bythe following equation:

Metal→Metal^(n+)+(n)e ⁻  (1)

The metal fuel may be of any type, and may be electrodeposited,absorbed, physically deposited, or otherwise provided on or constitutingthe fuel electrode. The fuel may be of any metal, including alloys orhydrides thereof, for example. For example, the fuel may comprisetransition metals, alkali metals, alkali earth metals, and other or“poor” metals. Transition metals include, but are not limited to zinc,iron, manganese, and vanadium. The most common alkali metal is lithiumbut other alkali metals may be used. The alkali earth metals include butare not limited to magnesium. The other metals include, but are notlimited to aluminum and gallium. As used herein, the term metal fuelrefers broadly to any fuel comprising a metal, including elementalmetal, metal bonded in a molecule, metal alloys, metal hydrides, etc.The fuel electrode may be formed of the metal fuel as the electrode bodyitself in some embodiments.

The fuel electrode may have any construction or configuration. Forexample, the fuel electrode may be a porous structure with athree-dimensional network of pores, a mesh screen, a plurality of meshscreens isolated from one another, or any other suitable electrode. Thefuel electrode includes a current collector, which may be a separateelement, or the body on which the fuel is received may beelectroconductive and thus also be the current collector. In anembodiment, the fuel electrode is laminated, bonded, or attached to abacking that provides the external surface of the fuel electrode. Thisbacking may be liquid impermeable or essentially impermeable to theionic liquid to prevent the ionic liquid from permeating outwardlythrough the fuel electrode via its external surface. The backing is alsoimpermeable to air, and particularly oxygen or other oxidant, to preventany undesirable parasitic reaction, such as oxidant reduction in thepresence of the fuel oxidation that occurs at the electrode duringdischarge. Further details regarding metal fuels and fuel electrodes maybe found in U.S. patent application Ser. Nos. 12/385,217, 12/385,489,12/776,962, 61/193,540, 61/329,278, and 61/243,970, the entirety of eachof which are incorporated herein.

The air electrode is the counter electrode. During discharge, oxygen atthe air electrode is reduced, consuming electrons. There are severalpossible mechanisms for oxygen reduction. The oxygen reduction reactionmay occur, for example, via one of the three mechanisms discussed below.Other mechanism, however, may occur depending on the chemical system(ionic liquid, electrode materials) chosen.

Oxygen Reduction Reactions

The oxygen reduction reactions can be highly dependent on the pH in anaqueous system. However, it is noted that at a high pH, the nature ofcatalyst, if present, becomes less important as that in a low pH.

It is believed an oxygen reduction reaction (“ORR”) in an aqueous systemcan take different forms, depending on the acidity of the system. Forexample, in an aqueous acid system, the reactions can be characterizedas:

O₂ ^(•−)(ads)+H⁺→HO₂ ^(•)(ads)

HO₂ ^(•)(ads)+H⁺ +e ⁻→H₂O₂,

H₂O₂+2H⁺+2e ⁻→2H₂O

with the net reaction being:

O₂+4H⁺+4e ⁻→2H₂O; E°=1.299V.

Several mechanisms in an acid system have been proposed. See Durand et.al., Electrochimica Acta (2003), Sawyer et al. 1981; Sawyer et al. 1981;Analytical Chemistry, vol. 54, pp. 1720 (1982); Honda, 1986. In someembodiments, a catalyst is provided to get useful rates. The catalystcan be, for example, silver and/or platinum. In one embodiment,superoxide pKa is about 4.7 in an aqueous system, but the conjugate basecan drive the reaction so completely that it behaves as if it has a pKaof 24. Thus, the reaction can proceed even in the presence of a veryweak acid, such as water.

On the other hand, in an aqueous basic (alkaline) system, the reactionscan be characterized as:

O₂ ^(•−)(ads)+H₂O→HO₂ ^(•)(ads)+OH⁻

HO₂ ^(•)(ads)+e ⁻→HO₂ ⁻,

HO₂ ⁻+H₂O+2e ⁻→3OH⁻

with the net reaction being

O₂+2H₂O+4e ⁻→4OH⁻; E°=0.401V.

Several mechanisms in an alkaline system have been proposed. See Ross,P. N., Handbook of Fuel Cells—Fundamentals, Technology and Applications,ch 31 (2003), Durand et. al., Electrochimica Acta (2003).

It is believed that the ORR in a dry protic system can have themechanism below:

O₂→O₂(ads)

O₂(ads)+e ⁻→O₂ ^(•) ⁻ (ads)

O₂ ^(•) ⁻ (ads)+BH⁺→HO₂ ^(•)(ads)+B

HO₂ ^(•)(ads)+e ⁻→HO₂ ⁻

HO₂ ⁻(ads)+BH⁺→H₂O₂+B

Further protonation of peroxidate by IL, as shown in the fifth reactionequation above, can produce peroxide, depending on pKa of BH⁺ (peroxidepKa=11.63). In principle, the reaction then proceeds as it may in anaqueous system:

H₂O₂ +e-→OH ⁻+OH^(•)(ads)

OH^(•)(ads)+e-→OH⁻

2OH⁻+2BH⁺→2H₂O+2B

Note that the protonation can depend on pKa of BH⁺.

A first possible and non-limiting mechanism is a four-electron ORR wherethe product is a fully reduced oxygen dianion. The four-electron oxygenreduction reaction may be represented by the following equation:

O₂+4e ⁻→2O²⁻  (2)

Depending on the specific chemistry of the system, this reaction mayform a soluble product or result locally in the formation of aninsoluble metal-oxide. An example of a four-electron oxygen reductionreaction Equation (3),

4AlCl+3O₂+12e ⁻→2Al₂O₃+16Cl⁻  (3)

In this reaction, the anions liberated may serve to mediate continuedanode reaction. Relative to the other oxygen reduction mechanisms, thefour-electron oxygen reduction reaction has the advantages of increasedenergy density and extracting the maximum number of electrons per oxygenmolecule. This mechanism, however, tends to have larger overpotentialsfor oxide dissociation during the oxygen evolution reaction (OER) duringrecharge, decreasing round-trip efficiency.

The second possible and non-limiting mechanism is a two-electronperoxide route. An example of this mechanism may be represented by thefollowing equation:

Zn²⁺+O₂+2e ⁻→ZnO₂  (4)

This mechanism has the advantage of relatively low overpotentials forthe peroxide reaction. It also tends to have enhanced rechargeabilityrelative to the first mechanism. The two-electron peroxide mechanism,however, has a lower energy density at the oxygen electrode relative toa four-electron process.

The third possible and non-limiting mechanism is a mixedtwo-electron/four-electron ORR that capitalizes on the reducing power ofcertain aliovalent cations. An example of this mechanism may berepresented by the following equation:

Mn²⁺+O₂+2e ⁻→MnO₂  (5)

The nuance in this mechanism is that the product involves fully reducedO²⁻ species generated by the reducing power of the aliovlant metal. Inthis example, Mn²⁺ ends up in the Mn⁴⁺ state on the right. Thismechanism has the advantage of lower overpotentials, measured to be aslarge as 350 mV or lower, due to the reducing power of aliovalentcations. Further, aliovalent metals may be used to make more efficientcells. The mixed two-electron/four-electron mechanism, however, has alower energy density at the oxygen electrode relative to a four-electronprocess.

FIG. 1 is a cyclic voltammogram of an air electrode half cellillustrating improvement in both the kinetics and thermodynamics of ametal ion-ionic liquid electrolyte in oxygen reduction according to anembodiment. In this embodiment, the electrolyte comprises oxygensaturated 1-methyl-3-octyl-imidazolium chloride with an addition of 6.1Mol % Mn²⁺. The comparative electrolyte is oxygen saturated1-methyl-3-octyl-imidazolium chloride without any added metal ions.

The top portion of FIG. 1 shows baseline cyclic voltammograms forAr-saturated (dearated) 1-methyl-3-octyl-imidizolium Chloride IL bothwith and without Mn(II) ions. The bottom portion of FIG. 1 shows thatthe addition of 6.1 Mol % Mn²⁺ results in a shift of approximately 260mV in the turn-on potential (the potential at which reduction starts)for oxygen reduction (O₂—saturated 1-methyl-3-octyl-imidizoliumChloride). That is, the half-cell potential for the O₂ reductionreaction shifts more positive relative to the reference electrode withthe addition of 6.1 Mol % Mn²⁺ to the oxygen saturated1-methyl-3-octyl-imidazolium chloride ionic electrolyte, increasing theoverall cell potential. In a Mn(II)-based battery, this improvementcorresponds to an approximate 14% increase in practical energy.

Note in FIG. 1 the magnitude of the shift in the voltammograms isillustrated with a shift in the half wave potentials A, B, the half-wavepotentials being easier to illustrate than the turn-on potentials. Thehalf-wave potential (E_(1/2)) is a potential at which the polarographicwave current is equal to one half of the diffusion current (i_(d)). Thatis, the potential, at which the current of a diffusion-controlledpolarographic wave reaches one half of the total wave height. For areversible redox-system, the half-wave potential is independent ofconcentration.

FIG. 1 also illustrates enhancement of the kinetics of reduction in theactivation regime. At the half-wave potential (approximately 1.5V) ofthe Mn containing ionic liquid electrolyte, the current density of theMn containing electrolyte is 2.9 times that of the comparativeelectrolyte at that same potential (approximately 1.5V). Additionally,the insert to FIG. 1 shows that the metal-oxide oxidation reaction ofthe oxygen saturated 1-methyl-3-octyl-imidazolium chloride—6.1 Mol %Mn²⁺ system is reversible.

In some embodiments, the metal of the added metal containing compound isthe same metal as that of the metal (fuel) electrode. In alternativeembodiments, the added metal is different from that of the metal (fuel)electrode. In one aspect, the solubility of the metal ions of the addedmetal compound may be larger than the solubility of the metal ions ofthe fuel electrode. In this aspect, the metal ions of the added metalcompound could promote oxygen reduction while the fuel metal oxideprecipitates preferentially. In still other embodiments, two or moredifferent metal containing compounds forming different metal ions areadded to the low temperature IL. In these embodiments, one of the metalsmay be, but need not be, the same metals as that of the metal (fuel)electrode metal.

When the cell starts to operate, the fuel electrode dissolves, addingmetal ions to the medium/solution. The thermodynamic and/or kineticadvantages of the added metal containing compound are sustained duringthe whole cycle. The initial quantity of an added metal containingcompound may be viewed as a “supporting salt” since the metal cationscarry along with them a set of anions (such as Cl⁻) that promote thecomplexation of cations from the anode. Also, once the saturation ofmetal ions is reached and precipitation of the metal oxide (peroxide,hydroxide, etc.) begins, the supporting ions can maintain a constantactivity of metal-centered ions in a solution.

FIG. 2 illustrates cyclic voltammagrams of additional embodiments.Specifically, FIG. 2 shows metal redox for (a) 0.01M Mn(II)acetate+triethylammonium methansulfonate, (b) 1MZnCl₂+diethylmethylammonium triflate, (c) 5MAlCl₃+1-butyl-3-methylimidazolium bis(trifluoromethane)sulfonamide, and(d) GaCl₃+1-methyl-3-octylimidazolium tetrachlorogallate (1:1). As canbe seen in FIG. 2, all of these systems show a high-degree ofreversibility.

In another non-limiting embodiment, the ionic liquid may betriethylammonium methanesulfonate (TEAMS) with 0.5 molar zinc triflatedissolved therein as an additive, and zinc may be used as the metalfuel. Potentiostatic studies of the half-cell reactions for zinc andoxygen in that ionic liquid indicate a cell potential of about 1.45V,and an estimated cell energy density in excess of 600 Wh/kg.Potentiostatic studies on the same TEAMS ionic liquid with 0.5 molarzinc triflate supplemented with 50 ppm water indicate a cell potentialof about 1.5V. In another non-limiting embodiment, the ionic liquid maybe TEAMS with 1.0 molar zinc bromide (ZnBr₂) dissolved as an additive,and zinc may be used as the metal fuel. Potentiostatic studies of thehalf-cell reactions for zinc and oxygen in that ionic liquid indicate acell potential of about 1.3V, an estimated cell energy density in excessof 500 Wh/kg, and a relatively high degree of reversibility for the zincand oxygen reactions, which is beneficial for secondary (rechargeable)cells.

In yet another non-limiting embodiment, the ionic liquid may bemethyloctylimidazolium chloride with 0.5 molar manganese (II) chloride(MnCl₂) and 50 ppm water as additives, and manganese may be used as themetal fuel. Potentiostatic studies of the half-cell reactions formanganese and oxygen in that ionic liquid indicate a cell potential ofabout 1.5V and an estimated cell energy density of about 800 Wh/kg. Instill another non-limiting embodiment, the ionic liquid may be1-butyl-3-methylimidazolium bis(trifluoromethane)sulfonamide with 5.0molar AlCl₃ as an additive, and aluminum may be used as the metal fuel.Potentiostatic studies of the half-cell reaction for aluminum in thationic liquid indicate a cell potential of about 2.5-2.8 V, an estimatedcell energy density of about 2500-3000 Wh/kg, and a relatively highdegree of reversibility for the aluminum reaction.

In yet another non-limiting embodiment, the ionic liquid may be diethylmethyl ammonium triflate (DEMATf) with 0.5 M ZnCl₂ dissolved therein asan additive, and zinc may be used as the metal fuel. This embodiment hasan estimated cell potential of about 1.3V. As still another non-limitingembodiment, the ionic liquid may be DEMATf with 0.5 M Zn(BF₄)₂ (zinctetrafluoroborate). This embodiment has an estimated cell potential ofabout 1.45V.

FIG. 3 illustrates a low temperature IL electrochemical cell(“electrochemical cell”), generally indicated at 10, according to someembodiments described herein. As illustrated and described below, theelectrochemical cell 10 includes a plurality of electrodes including afirst electrode 12 and a second electrode 14. In other embodiments, thefirst electrode or the second electrode of the electrochemical cell 10may be provided by configurations other than a single electrode. Thus,the use of a single electrode as presented in FIG. 1 for each of thefirst electrode 12 and the second electrode 14 is not intended to belimiting. In the non-limiting embodiment illustrated in FIG. 1, thefirst electrode 12 is a cathode, and more specifically an air cathode,and will be referred to hereinafter as an air electrode 12. The secondelectrode 14 is an anode, and will be referred to hereinafter as a metalelectrode 14. In an embodiment, and as described below, theelectrochemical cell 10 may generate electricity by virtue of anoxidation half-reaction of a fuel at the metal electrode 14 in parallel,that is, substantially at the same time, with a reduction half-reactionof an oxidizer 20 at the air electrode 12. The illustrated embodiment isnot intended to be limiting in any way.

As shown in FIG. 3, and as discussed in further detail below, the airelectrode 12 and the metal electrode 14 are spaced to form a gap 16therebetween. An RTIL, generally indicated at 18, may flow along the gap16 so that the low temperature IL 18 may contact both the air electrode12 and the metal electrode 14 at the same time. In an embodiment, itshould be understood that the electrochemical cell 10 may be oriented inany way, and the low temperature IL may flow in directions other thanwhat is illustrated. Thus, any directional references are made withregard to the orientation as shown in FIG. 1, and are not intended tolimit a working embodiment to any particular orientation. In otherembodiments, the low temperature IL 18 may be static with no flow atall. The low temperature IL 18 may make contact with the air electrode12 at an air electrode/low temperature IL interface 24. The lowtemperature IL 18 may make contact with the metal electrode 14 at ametal electrode/low temperature IL interface 26. In alternativeembodiments, the low temperature IL does not flow. That is, no mechanismfor forced flow is included in the cell.

As alluded to above, a reduction half-reaction may take place at the airelectrode 12. In an embodiment, an oxidizer 20 may be reduced throughthe reduction half-reaction at the air electrode 12. For non-limitingillustration purposes, the electrons from the metal electrode 14 mayflow to an external circuit 22 (i.e., a load) and return to the airelectrode 12 to facilitate the reduction of the oxidizer 20. Theoxidizer 20 is reduced on the air electrode 12 at oxidizer reductionreaction sites 21. In an embodiment, a catalyst is used to facilitatethe oxidizer reduction half-reaction at the oxidizer reduction reactionsites 21. The air electrode 12 may include catalyst material, such asmanganese oxide, nickel, pyrolized cobalt, activated carbon, silver,platinum, or any other catalyst material or mixture of materials withhigh oxygen reduction activity for catalyzing reduction of the oxidizer,which will be discussed below. In an embodiment, the air electrode 12may be porous and the porous body with a high surface area may comprisethe catalyst material.

In an embodiment, the air electrode 12 may be a passive or “breathing”air electrode 12 that is passively exposed, such as through windows oropenings to an oxidizer source (typically oxygen present in ambient air)and absorbs the oxidizer 20 for consumption in the electrochemical cell10 reactions. That is, the oxidizer 20 will permeate from the oxidizersource into the air electrode 12. Thus, the oxidizer 20 need not beactively pumped or otherwise directed to the air electrode 12, such asvia an inlet. Any part of the air electrode 12 by which the oxidizer 20is absorbed or otherwise permeates or contacts the air electrode 12 maybe generically referred to as an “input.” The term input may broadlyencompass all ways of delivering oxidizer to the air electrode 12 forthe oxidizer reduction half-reaction at the oxidizer reduction reactionsites 21 on the air electrode 12.

By means of a non-limiting illustration, the air electrode 12 may be agas permeable electrode having an outer surface exposed to ambient airsuch that the oxidizer 20 comprises oxygen that permeates the airelectrode 12. Similarly, the air electrode 12 may comprise a barriermembrane on the outer surface of the air electrode 12 that is gaspermeable and liquid impermeable so as to permit permeation of theoxidizer 20 via the outer surface of the air electrode 12 and preventthe low temperature IL 18 from flowing through the outer surface of theair electrode 12. In an embodiment, the air electrode 12 may be a porousbody covered on the inner side by a liquid permeable layer through whichthe low temperature IL 18 may pass through so that the low temperatureIL 18 may contact the porous body.

The relationship between the low temperature IL 18 and the air electrode12 may impact the overall energy density of the electrochemical cell 10.For that reason, the vapor pressure and surface tension characteristicsof the low temperature IL 18 in view of the air electrode 12 may becarefully selected. For instance, in an embodiment, the air electrode 12may repel the low temperature IL so that it may prevent the lowtemperature IL 18 from wicking; that is, flowing in a capillary-likemanner through the air electrode 12. In another embodiment, the airelectrode 12 may be designed with porosity to absorb the low temperatureIL so that it exposes the low temperature IL to more air electrode 12surface area for purposes of enabling the desired electrochemicalreactions at the air electrode 12. The air electrode 12 may supportcatalyst decoration at the oxidizer reduction reaction sites 21 toimprove the efficiency of the reaction. In an embodiment, the catalystmay be decorated with metal ions which may enhance the activity of thecatalyst in catalyzing the oxidizer reduction reaction at the oxidizerreduction reaction sites 21 on the air electrode 12. The air electrode12 may have a high ionic conductivity to provide reactants and removeproducts of the oxidizer reduction reaction from the air electrode 12.In an embodiment, the air electrode 12 may have high electricalconductivity characteristics to carry electrons from the external load22 to the oxidizer reduction reaction sites 21. The air electrode 12 andlow temperature IL 18 characteristics may be further defined.

In an embodiment, the metal-oxide by-products 28 may be formed at themetal electrode 14. Whereas reduced oxidizer ions in an aqueouselectrolyte coordinate, that is, donate electrons to water molecules toform water, peroxides and/or hydroxides, and thereby increase problemswith vapor pressure and corrosion. In this non-limiting embodiment, thelow temperature IL 18 may promote both the oxidizer reduction reactionat the air electrode 12 and the conduction of the reduced oxidizer ionsto the metal electrode 14. In support of this result, the lowtemperature IL 18 may contain soluble species that interact with thereduced oxidizer ions, with the low temperature IL 18 typically beingprotic. The low temperature IL 18 may also support the reduced oxidizerions as they migrate to the metal electrode 14. By means of anon-limiting illustration, the migration of the reduced oxidizer ionsmay refer to transport of the reduced oxidizer ions via convectiontransport, or conduction transport or diffusion transport. The lowtemperature IL 18 may also support the oxidized metal-fuel ionsremaining at the metal electrode 14. In doing so, the low temperature IL18 promotes the reaction between the reduced oxidizer ions and theoxidized metal-fuel ions to produce the metal-oxide by-products 28. Inan embodiment, the metal-oxide by-products 28 may be stored at the metalelectrode 14. In an embodiment where the metal-oxide by-product 28 isstored at the metal electrode 14, this embodiment is best used as aprimary (i.e., non-rechargeable) battery, as the oxygen is stored at themetal electrode 14 and is not locally available to an oxygen evolvingelectrode for oxidation of the reduced oxygen species.

In another embodiment, the metal-oxide by-products 28 may be formed atthe air electrode 12. In this non-limiting embodiment, the air electrode12 catalyzes the oxidizer reduction reaction at the oxidizer reductionreaction sites 21 at the air electrode 12. In an embodiment, the lowtemperature IL 18 (typically aprotic) may be chemically compatible withpure metal or metal alloy, and high concentrations of the oxidizedmetal-fuel ions may exist in the low temperature IL 18. In anotherembodiment, metal ions are added to the electrolyte, which formsmetal-oxide by-products 28 at the air electrode 12. As discussed above,the added metal ions may or may not be of the same metal as the metalelectrode. In another embodiment, the metal-oxide by-products 28 arestored locally at the air electrode 22. Because metal-oxide by-products28 are formed and stored locally at the air electrode 12 duringdischarge, a ready supply of oxygen (present in the locally stored metaloxide) is locally available at the air electrode during recharge. Inthis manner, the reversibility of the cell can be improved. In contrast,where the oxides are stored in the ionic liquid electrolyte, the oxidesare typically distributed throughout the electrolyte, and the amount ofoxide available to the air electrode is limited to the electrolyte/airinterface and rate at which the oxide can diffuse within the electrolyteto that interface.

The storage of the metal oxide locally at the air electrode isfacilitated by the air electrode 12 having a pore size in at least theregions contacting the ionic liquid sufficient to contain the oxidewithin the air electrode 12 body. That is, the pore size may bedependent on the size of the oxide. A network of such pores may increasethe storage capacity of the air electrode 12.

In another embodiment, the low temperature IL 18 may support solvatingthese oxidized metal-fuel ions at the metal electrode 14. That is, thelow temperature IL ions may surround the metal-fuel ions, and in doingso, the low temperature IL 18 may help to maintain the metal-fuel ionicform as the solvated, oxidized metal-fuel ions migrate to the airelectrode 12. Typically, the low temperature IL will be aprotic. Bymeans of a non-limiting illustration, the migration of the solvated,oxidized metal-fuel ions may refer to transport of the solvated,oxidized metal-fuel ions via convection transport, or conductiontransport or diffusion transport. Once at the air electrode 12, thesolvated metal-fuel ions may react with the reduced oxidizer ions, andthis reaction may result in metal-oxide by-products 28. In anembodiment, the metal-oxide by-products 28 may be stored at the airelectrode 12.

In an embodiment, the metal-oxide by-product 28 may catalyze theoxidizer reduction reaction at the air electrode 12. In an embodiment,the electrochemical cell 10 may include a regenerative electrochemicalcell and an oxygen recovery system. Examples of such devices are shown,for example, in U.S. patent application Ser. No. 12/549,617, filed onAug. 28, 2009, which is incorporated herein by reference in itsentirety.

In an embodiment, the oxidizer source is ambient air, and the oxidizer20 is oxygen. In an embodiment, oxygen as the oxidizer 20 may be reducedat the air electrode 12 to form reduced oxygen ions. In an embodiment,the oxygen may be supplied from an evolved oxygen recovery system usedin a regenerative electrochemical cell. Other examples ofelectrochemical cells that may be useful embodiments herein are shown,for example, in U.S. patent application Ser. No. 12/549,617, filed onAug. 28, 2009, which is incorporated herein by reference in itsentirety.

The electrolytes and/or ionically conductive medium described herein maybe used in other cell configurations. An alternate cell configuration,for example, comprises a compact wound cell illustrated in copendingU.S. patent application Ser. Nos. 61/267,240, filed Dec. 7, 2009 and12/776,962, filed May 10, 2010, which are hereby incorporated byreference in their entirety. All the layers (i.e., the electrodes andelectrolyte layers or layers) may be flexible (either by being aflexible solid or semi-solid or being a liquid which is inherentlyconformable and thus may be considered flexible) to enable the cell tobe configured in a wound, folded or other non-linear arrangement withthe outer surface of the air electrode exposed for absorbing oxygen. Aflexible, insulating air-permeable separator may be positioned betweenthe outer surface of the fuel and air electrodes to maintain spacingtherebetween and permit air to permeate to the outer surface of the airelectrode. The separator may be of any configuration, including alattice, ribbed structure, etc.

FIG. 4 illustrates a method according to an embodiment. This embodimentincludes the steps of mixing metal ions with an ionic liquid to create asolution comprising a metal ion-negative ion complex 102, exposing thesolution to oxygen 104 and electrochemically reducing the oxygen 106.Optionally, the process may include one or more of the following steps:forming metal-oxide by-products at a metal fuel electrode 108, storingthe metal-oxide by-products at the metal electrode 110, forming ametal-oxide by-product at an air electrode 112, or storing themetal-oxide by-products at the air electrode 114.

Thus, it can be seen that an additive in the low or room temperatureionic liquid may provide cations (positive ions) for enhancing theoxygen reduction reaction in various ways. As mentioned above, thecation may be a metal ion that coordinates with one or more negativeions in the ionic liquid to form a metal ion-negative ion complex thatimproves oxygen reduction thermodynamics and/or kinetics, or promotesthe formation and storage of oxides of the metal fuel at the airelectrode. Under conditions where ORR is occurring, a high concentrationof metal ions near and within the cathode provides for the complexing ofthe reduced oxygen species and the subsequent precipitation of thosespecies, thereby promoting the formation and storage of oxides at theair electrode. The additive may also be water, which provides itspositive ions (H⁺) for the same purposes. Water may be added in advance,or may be absorbed as water vapor through the air electrode via itsnatural absorption characteristics. Thus, in embodiments where theadditive improves oxygen reduction thermodynamics and/or kinetics, theadditive may be referred to as an oxygen reduction enhancing additive,which could be a metal containing additive, water, or another additivethat provides a positive cation for coordinating with the negative ionof the ionic liquid for the same purpose. Similarly, in embodimentswhere the additive promotes the formation and storage of oxides of themetal fuel at the air electrode during discharge, the additive may bereferred to as a local oxide formation promoting additive, which alsomay be a metal containing additive, water, or another additive thatprovides a positive cation for coordinating with the negative ions ofthe ionic liquid for the same purpose. The functionality of an oxygenreduction enhancing additive and a local oxide formation enhancingadditive are not mutually exclusive, and the same additive may provideboth functionalities. Accordingly, a local oxide formation enhancingadditive can also be an oxygen reduction enhancing additive.

Whether water content is desired in the ionic liquid may be dictated bythe reactivity of the metal fuel used, particularly if the ionic liquidcontacts the fuel electrode. For example, if the metal is highlyreactive, in some embodiments water may cause self-discharge (i.e.,metal oxidation and hydrogen evolution at the fuel electrode), and thusit may be preferred to avoid or minimize any water content and have theionic liquid be highly hydrophobic. Such highly reactive metals may bethose metals in Groups I-VI, XIII and XIV of the periodic table. Forless reactive metals, such as those in Groups VII-XII of the periodictable, the benefits of water content may outweigh the negatives in someembodiments and thus water content may be used. An advantage of watercontent is that it can be replenished by absorption of water vaporthrough the air electrode, whereas a metal additive may precipitate outover extended periods of time and require replenishment.

In various embodiments the ionically conductive medium between the fueland air electrodes (and the charging electrodes, if a separate one isused) may have multiple layers, instead of having the ionic liquidcontact each of the electrodes as illustrated. For example, two ionicliquids separated by an interface, such as a membrane, or an ionicliquid and a semi-solid electrolyte may be used. Other configurationsmay be used as well.

Additives in IL

As described above, the ionically conductive medium can comprise atleast one oxygen reduction enhancing compound. In one embodiment, such acompound can be seen as an “additive,” as its amount is generallysmaller than the amount of the ionic liquid(s) in the ionicallyconductive medium. For example the ratio of the concentration of theadditive to ionic liquid can be at least about 1:1000, such as at leastabout 1:500, such as at least about 1:100, such as at least about 1:10,such as at least about 1:5, such as at least about 1:1.

In some embodiments, such a compound can also be a (local) oxideformation promoting compound, as it can increase the formation andstorage of oxides of the metal fuel at the air electrode duringdischarge relative to the ionic liquid without the local oxide formationpromoting compound. In one embodiment, a local oxide formation promotingcompound/additive can be added to an ionically conductive medium, theadditive dissolving in the medium to form local oxide formationpromoting positive ions. Similar to an oxygen reduction enhancingcompound, the positive ions of the local oxide formation promotingcompound are coordinated with one or more negative ions forming localoxide formation promoting positive ion-negative ion complexes. Theoxygen reduction enhancing compound described herein can be any of suchcompounds aforedescribed.

One surprising advantage of adding an oxygen reduction enhancingcompound is to improve oxygen reduction thermodynamics, kinetics, orboth. In one embodiment, the compound dissociates into positive ionsthat coordinate with at least one negative ion to form apositive-negative ion complex. The complex can have a large effect onthe turn-on potential, half-wave potential, and/or the current densityat the half-wave potential. Depending on the additive, the reversibilityof the reaction may also be affected.

For example, in one embodiment, the oxygen reduction enhancingpositive-negative ion complex can produce a shift of greater than orequal to about 200 mV in the turn-on potential for oxygen reductionversus the ionic liquid without the added oxygen reduction enhancingcompound. The shift can be greater than or equal to about 100 mV, about150 mV, about 240 mV, about 300 mV, about 350 mV, about 400 mV, about500 mV, or more.

In one embodiment, the oxygen reduction enhancing positive-negative ioncomplex can produce an increase in the current density at the half-wavepotential versus the ionic liquid without the oxygen reduction enhancingcompound. The current density can also be maintained at substantiallythe same value before the addition of the additive.

In another embodiment, the oxygen reduction enhancing positive-negativeion complex can produce a shift of greater than or equal to about 1 V inthe half-wave potential for oxygen reduction versus the ionic liquidwithout the added oxygen reduction enhancing compound. The shift can begreater than or equal to about 0.1 V, about 0.5 V, about 1 V, about 1.5V, about 2 V, about 2.5 V, or more.

Depending on the additive, the proton in an additive may improve oxygenreduction thermodynamics, kinetics, or both, relative to the ionicallyconductive medium without the protic ionic liquid. In one embodiment,the presence of a proton can increase the formation and storage ofoxides of the metal fuel at the air electrode during discharge relativeto the ionically conductive medium without the protic ionic liquid.

The ionically conductive medium in the aforedescribed electrochemicalcell can comprise more than one IL. In one embodiment, the ionicallyconductive medium can comprise at least one aprotic IL and at least oneprotic liquid that comprises at least one available proton per ion pair.For example, the medium can comprise one, two, three, four, or more,aprotic ILs, alone or together with one, two, three, four, or more,protic ILs. The protic IL can comprise at least one available proton perion pair. For example, in a case wherein at least one protic IL is addedto an aprotic IL, because the amount of the protic IL is smaller thanthat of the aprotic, the protic IL can be seen also as an additive, asdescribed above.

The two (or more) ionic liquids can be mixed together to create anionically conductive medium, which can be one of those described abovein an electrochemical cell. In particular, the medium is exposed togases, such as oxygen gas, to allow electrochemical reduction of oxygento begin. Note that an oxygen reduction enhancing compound can besimilarly mixed with at least one ionic liquid to form an ionicallyconductive solution/medium as well.

The amount of the protic and aprotic ILs in the ionically conductivemedium can vary depending on the chemistry of the medium. For examplethe ratio of the concentration of the protic ionic liquid to the proticionic liquid is at least about 1:1000, such as at least about 1:500,such as at least about 1:100, such as at least about 1:10, such as atleast about 1:5, such as at least about 1:1. Alternatively, the amountof protic IL can be larger than that of the aprotic IL. For example, theratio of protic to aprotic ILs can be at least 1:0.5, such as at least1:0.1, such as at least 1:0.05, such as at least 1:0.01.

As aforedescribed, the protic IL can comprise at least one cation thatcomprises at least one reversible proton. The pKa of the proton of theprotic source (additive) can depend on the protic source. The term “pKa”can be appreciated by one of ordinary skill in the art to refer tologarithmic measure of the acid dissociation constant. A strong acid,for example, can have a pKa value of less than about −2. In oneembodiment, the protic ionic liquid comprises at least one cationcomprising at least one reversible proton with a pKa smaller or equal to16, such as smaller or equal to 14, such as smaller or equal to 12, suchas smaller or equal to 10, such as smaller or equal to 8, such assmaller or equal to 6, such as smaller or equal to 4, such as smaller orequal to 2. The pKa can also be a negative number. For example, the pKacan be smaller than or equal to −2, such as smaller or equal to −4, suchas smaller or equal to −6, such as smaller or equal to −8, such assmaller or equal to −10, such as smaller or equal to −12.

The descriptions provided herein can be readily applied to a Lewis acidsystem, which may drive cell chemistry for highly reactive metals. Notethat the embodiments provided herein can also be applied to a Lewis basesystem.

NON-LIMITING WORKING EXAMPLES

A series of protic additives were added into an ionically conductivemedium in order to examine the effects of such additives upon oxygenreduction reaction (“ORR”). An aprotic ionic liquid BdMelm:Tf was usedas the host for the additive. A photograph of an experimental setup isprovided in FIG. 5( a), with FIG. 5( b) showing a cartoon of a close-upof the measurement tube/chamber. This particular IL was chosen for itselectrochemical air, and water stability of its ions and that theprotonation of superoxide in BMlm was observed even in very dry systems.For example, BDMelm Tf has a melting point of about 35° C., density ofabout 1.4 g/mL, and oxygen solubility of about 2.5 mM, as measuredgravimetricallty and through Cottrell, diffusivity of about 7.5×10-6cm2/s as measured for dry ORR on glassy carbon (GC) and with Cottrell.See e.g., FIG. 5( f). Also, the IL had the benefit of the superacidityof Tf, which minimizes solvent leveling effects. Additional benefits ofthis dry aprotic IL system include also invariant electrode, highreversibility, and high reactivity, as shown in cyclic voltammagrams(CV) in FIG. 5( c) and the result of the Koutecky-Levich (“L-K”)analysis with respect to the number of electrons per O₂ vs. potential inFIG. 5( d). Additional background ORR analysis data regarding linearsweep voltammetry (LSV) at 10 mV/s in N₂ deaerated with 62 ppm water forBDMelm:Tf (to be used as blank, control data) are shown in FIG. 5( e).

A range of protic additives were then titrated, and electrochemical ORRanalysis on each system was subsequently performed. Table 1 shows asummary of the results related to these additives. As shown in Table 1,the pKa value of the series ranged from about 16 to about −14. These arealso shown on FIG. 6.

It was found that the reactions involving triflic acid (HTf),benzonitrile: HTf, acetophenone: HTf, methanesulfonic acid, hydroniumtriflate, pyridazinium triflate involved a four-electron transfer. Thereactions involving acetic acid, pyridinium triflate,1,2-dimethylimidaozlium triflate, n,n-diethyl-n-methylammonium triflateinvolved a two-electron transfer. Finally, the reaction involving waterinvolved an one-electron system.

TABLE 1 pKa values of different Proton Sources Proton Source pKa (H₂O)triflic acid (“HTf”) −14 benzonitrile: HTf −10 acetophenone: HTf(“MAcPh”) −6.2 methanesulfonic acid “(HMeS”) −2.6 hydronium triflate(“H₃O⁺ ”) −1.7 pyridazinium triflate 2.1 acetic acid 4.76 pyridiniumtriflate (“PyrH⁺”) 5.21 1,2-dimethylimidaozlium triflate (“dMImH⁺”) 7.4n,n-diethyl-n-methylammonium triflate 10.6 2,2,2-trifluoroethanol(“TFEtOH”) 12.5 2-butyl-1,1,3,3-tetramethylguanidinium triflate 13.6water 15.7

FIGS. 7( a)-(b) show cyclic voltammograms of the embodiment with wateras an additive on Pt (7(a)) and glassy carbon (“GC”) disk (7(b)) at 50°C. A series of water content was studied, with the highest concentrationbeing about 850 mM (about 1 H₂O per 5 BdMeIm Tf). FIGS. 7( c)-(d) showadditional data regarding LSV at 10 mV/s on both Pt disk (7(c)) and GCdisk (7(d)) at 50° C. FIGS. 7( e)-(f) show the results of L-K analysiswith respect to the number of electrons per O₂ vs. potential for Pt disk(7(e)) and GC disk (7(f)). It was found that even at largeconcentrations of water, superoxide is dominant.

FIGS. 8( a)-(c) show cyclic voltammogram of the embodiment with HTf asthe protic additive. It was found that there was a shift of greater than1V in at the half-wave potential for the oxygen reduction reaction. Aloss of reversibility was also seen. FIGS. 8( d)-(e) show the results ofthe K-K analysis. It was observed that on Pt disk a four-electronreaction was involved when HTf is 100 mM, while on GC disk, atwo-electron reaction in the first reduction was involved.

FIGS. 9( a)-(b) show cyclic voltammogram of the embodiment with HMeS asthe protic additive. FIGS. 9( c)-(d) show the results of the L-Kanalysis. It was found that the results were similar to those as shownin FIG. 8. There was a shift of greater than 1V in at the half-wavepotential for the oxygen reduction reaction. A loss of reversibility wasalso seen.

FIGS. 10( a)-(b) show cyclic voltammogram of the embodiment with HMeS asthe protic additive. FIGS. 10( c)-(d) show the results of the L-Kanalysis. It was observed from the L-K analysis that a weak acidproduced a four-electron ORR in both Pt and GC disk. FIGS. 11( a)-(b)show the results of the L-K analysis in the embodiment with TFEtOH asthe protic additive. It is seen from the Figure that only ORR involvingtwo-electron transfer was observed.

FIG. 12 shows the solvent leveling effects as observed in the oxygenreduction reaction onsets versus pKa of the protic additive in water forboth Pt and GC disks. As shown in FIG. 11, the Pt generally had a higherORR onset voltage than GC. Also, the ORR onset voltages increase withincreasing pKa, but appeared to level off at least for GC when pKareaches between 0 and −5.

FIGS. 13( a)-(b) show the oxygen reduction reaction turn-on potential asa function of pKa for Pt and GC. It was noted that there was 100 mVerror in estimated RE shift between deaerated and O₂ saturated IL withhydrogen redox potential. Hydrogen potential was defined was(E_(1/2)+E_(1/2))/2 for cathodic and anodic sweeps. ORR turn-onpotential was taken from Tafel plot CV at 100 m/s and 1600 rpm duringcathodic sweep.

The results show that oxygen reduction reaction was tunable to productsthat range from superoxide (neat, water, and low protic additivecontents), to peroxide (TFEtOH), to four-electron with more acidicspecies. It was found that the protic additive source and pKa affectedthe reaction mechanism of the oxygen reduction reactions. It was foundthat increasing the amount of water as an additive (up to ˜1M) in thisexperiment appears to have little effect on the oxygen reductionreaction behavior. It was postulated that hydrogen-bonding at theseconcentrations (still only ⅕ per ion pair) prevented it from acting asbulk water. Organic salts were found to have an impact at much lowerconcentrations. Finally, no observable change was observed for pKa'sbelow −2.5, suggesting substantial solvent leveling effects in theBdMeIm Tf IL.

The foregoing description has been presented for purposes ofillustration and description. It is not intended to be exhaustive or tolimiting to the precise form disclosed, and modifications and variationsare possible in light of the above teachings or may be acquired frompractice of the invention. The drawings and description were chosen inorder to explain the principles of the invention and its practicalapplication. It is intended that the scope of the invention be definedby the claims appended hereto, and their equivalents.

The articles “a” and “an” are used herein to refer to one or to morethan one (i.e., to at least one) of the grammatical object of thearticle. By way of example, “a polymer resin” means one polymer resin ormore than one polymer resin. Any ranges cited herein are inclusive. Theterms “substantially” and “about” used throughout this Specification areused to describe and account for small fluctuations. For example, theycan refer to less than or equal to +5%, such as less than or equal to+2%, such as less than or equal to ±1%, such as less than or equal to±0.5%, such as less than or equal to ±0.2%, such as less than or equalto ±0.1%, such as less than or equal to ±0.05%.

1. An electrochemical cell comprising: a fuel electrode for oxidizing ametal fuel; a low temperature ionic liquid comprising positive ions andnegative ions; an oxygen reduction enhancing compound added to the lowtemperature ionic liquid to form oxygen reduction enhancing positiveions, wherein the oxygen reduction enhancing positive ions arecoordinated with one or more negative ions forming oxygen reductionenhancing positive-negative ion complexes; and an air electrodeconfigured to absorb and reduce oxygen, wherein the oxygen reductionenhancing positive-negative ion complex improves oxygen reductionthermodynamics, kinetics, or both, relative to the ionic liquid withoutthe oxygen reduction enhancing compound.
 2. The electrochemical cell ofclaim 1, wherein the oxygen reduction enhancing compound comprises ametal-containing additive.
 3. The electrochemical cell of claim 1,wherein the oxygen reduction enhancing compound comprises a metal,water, or both.
 4. The electrochemical cell of claim 1, wherein theoxygen reduction enhancing compound comprises an organic moleculecontaining additive.
 5. The electrochemical cell of claim 1, wherein theoxygen reduction enhancing compound comprises a protic organic moleculecontaining additive.
 6. The electrochemical cell of claim 1, wherein theoxygen reduction enhancing compound comprises an additive that is atleast one of triflic acid, benzonitrile: HTf, acetophenone: HTf,methanesulfonic acid, hydronium triflate, pyridazinium triflate, aceticacid, pyridinium triflate, 1,2-dimethylimidaozlium triflate,n,n-diethyl-n-methylammonium triflate, 2,2,2-trifluoroethanol,2-butyl-1,1,3,3-tetramethylguanidinium triflate, and water.
 7. Theelectrochemical cell of claim 1, wherein the ionic liquid comprises atleast one aprotic ionic liquid comprising at least one cation that hasat least one strongly bound proton.
 8. The electrochemical cell of claim1, wherein the oxygen reduction half-reaction involves a transfer of atleast two electrons per oxygen molecule.
 9. The electrochemical cell ofclaim 1, wherein the oxygen reduction half-reaction involves a transferof at least four electrons per oxygen molecule.
 10. The electrochemicalcell of claim 1, wherein the low temperature ionic liquid is selectedfrom the group consisting of triethylammonium methanesulfonate,1-methyl-3-octylimidazolium tetrachlorogallate, diethylmethylammoniumtriflate, and 1-butyl-3-methylimidazoliumbis(trifluoromethane)sulfonamide.
 11. The electrochemical cell of claim1, wherein the low temperature ionic liquid is hydrophobic.
 12. Theelectrochemical cell of claim 1, wherein the air electrode comprisespolytetrafluoroethylene (PTFE).
 13. The electrochemical cell of claim 1,wherein the fuel electrode is porous.
 14. The electrochemical cell ofclaim 1, wherein the low temperature ionic liquid is aprotic.
 15. Theelectrochemical cell of claim 1, wherein the low temperature ionicliquid comprises a room temperature ionic liquid (RTIL).
 16. Theelectrochemical cell of claim 1, wherein the low temperature ionicliquid comprises a vapor pressure at or below 1 mm Hg at 20° C. aboveits melting point.
 17. The electrochemical cell of claim 1, wherein thelow temperature ionic liquid comprises an anion selected from the groupconsisting of tetrachloroaluminate, bis(trifluoromethylsulfonyl)imide,methylsulfonate, nitrate, and acetate.
 18. The electrochemical cell ofclaim 1, wherein the low temperature ionic liquid comprises a cationselected from the group consisting of imidazolium, sulfonium,pyrrolidinium, pyridinium, triethylammonium, diethylmethylammonium,dimethylethylammonium, ethylammonium, α-picolinium,1,8-bis(dimethylamino)naphthalene, 2,6-di-tert-butylpyridine,quaternized ammonium and phosphonium.
 19. The electrochemical cell ofclaim 1, wherein the fuel electrode further comprises a liquidimpermeable backing
 20. The electrochemical cell of claim 1, wherein thefuel electrode further comprises a liquid impermeable backing that isfurther impermeable to air.
 21. The electrochemical cell of claim 1,wherein the oxygen reduction enhancing positive-negative ion complexesproduce a shift of greater than or equal to about 200 mV in the turn-onpotential for oxygen reduction versus the ionic liquid without the addedoxygen reduction enhancing compound.
 22. The electrochemical cell ofclaim 1, wherein the oxygen reduction enhancing positive-negative ioncomplexes produces an increase in the current density at the half-wavepotential versus the ionic liquid without the oxygen reduction enhancingcompound.
 23. The electrochemical cell of claim 1, wherein the oxygenreduction enhancing positive-negative ion complexes produce a shift ofgreater than or equal to about 1 V in the half-wave potential for oxygenreduction versus the ionic liquid without the added an oxygen reductionenhancing compound.
 24. The electrochemical cell of claim 1, wherein theair electrode comprises a catalyst.
 25. The electrochemical cell ofclaim 1, wherein the air electrode comprises a catalyst that is selectedfrom the group consisting of manganese oxide, nickel, pyrolized cobalt,porphyrin-based catalysts, activated carbon, perovskites, spinels,silver, platinum, and/or mixtures thereof.
 26. The electrochemical cellof claim 1, wherein the air electrode is gas permeable.
 27. Theelectrochemical cell of claim 1, wherein the air electrode comprises abarrier membrane on an outer surface of the air electrode, and whereinthe barrier membrane is gas permeable and liquid impermeable.
 28. Theelectrochemical cell of claim 1, wherein the air electrode repels thelow temperature ionic liquid.
 29. The electrochemical cell of claim 1,wherein metal-oxide by-products are formed at the fuel electrode. 30.The electrochemical cell of claim 1, wherein the fuel electrode isporous and wherein metal-oxide by-products are stored at the fuelelectrode.
 31. The electrochemical cell of claim 1, wherein metal-oxideby-products are formed at the air electrode.
 32. The electrochemicalcell of claim 1, wherein metal-oxide by-products are stored at the airelectrode.
 33. The electrochemical cell of claim 1, wherein the ionicliquid comprises a protic ionic liquid comprising at least one cationcomprising at least one reversible proton.
 34. The electrochemical cellof claim 1, wherein the ionic liquid comprises a protic ionic liquidcomprising at least one cation comprising at least one reversible protonwith a pKa smaller or equal to
 16. 35. The electrochemical cell of claim1, wherein the ionic liquid comprises a protic ionic liquid comprisingat least one cation comprising at least one reversible proton with a pKasmaller or equal to
 14. 36. A method comprising: mixing an oxygenreduction enhancing compound with a low temperature ionic liquid tocreate a solution comprising an oxygen reduction enhancingpositive-negative ion complex; exposing the solution to oxygen; andelectrochemically reducing the oxygen.
 37. The method of claim 36,wherein the reducing the oxygen occurs with improved oxygen reductionthermodynamics, kinetics, or both, relative to electrochemical oxygenreduction in the ionic liquid without the metal-containing additive. 38.The method of claim 36, wherein the oxygen is electrochemically reducedusing a catalyst.
 39. The method of claim 36, wherein theelectrochemically reducing the oxygen occurs in an electrochemical cell.40. The method of claim 36, wherein the oxygen reduction enhancingcompound comprises a metal-containing additive.
 41. The method of claim36, wherein the oxygen reduction enhancing compound comprises ametal-containing additive, the metal of which is selected from the groupconsisting of Mg, Al, Mn, Ga, and Zn.
 42. The method of claim 36,wherein the oxygen reduction enhancing compound comprises a metal,water, an organic molecule, or combinations thereof.
 43. The method ofclaim 36, wherein the oxygen reduction enhancing compound comprises aprotic organic molecule containing additive.
 44. The method of claim 36,wherein the oxygen reduction enhancing compound comprises an additivethat is at least one of triflic acid, benzonitrile: HTf, acetophenone:HTf, methanesulfonic acid, hydronium triflate, pyridazinium triflate,acetic acid, pyridinium triflate, 1,2-dimethylimidaozlium triflate,n,n-diethyl-n-methylammonium triflate, 2,2,2-trifluoroethanol,2-butyl-1,1,3,3-tetramethylguanidinium triflate, and water.
 45. Themethod of claim 36, wherein the low temperature ionic liquid is selectedfrom the group consisting of triethylammonium methansulfonate,1-methyl-3-octylimidazolium tetrachlorogallate, diethylmethylammoniumtriflate, and 1-butyl-3-methylimidazoliumbis(trifluoromethane)sulfonamide.
 46. The method of claim 36, whereinthe electrochemically reducing the oxygen occurs in a metal-air ionicliquid battery comprising a metal electrode and an air electrode. 47.The method of claim 36, wherein the oxygen reduction enhancingpositive-negative ion complex enhances reversibility of an air cathodeof a metal-air ionic liquid battery.
 48. The method of claim 36, whereinthe low temperature ionic liquid is aprotic.
 49. The method of claim 36,wherein the low temperature ionic liquid is a room temperature ionicliquid (RTIL).
 50. The method of claim 36, further comprising flowingthe low temperature ionic liquid in a gap between a metal electrode andan air electrode.
 51. The method of claim 36, further comprising formingmetal-oxide by-products at a metal fuel electrode.
 52. The method ofclaim 36, further comprising storing metal-oxide by-products at themetal electrode.
 53. The method of claim 36, further comprising formingmetal-oxide by-product at an air electrode.
 54. The method of claim 36,further comprising storing metal-oxide by-products at the air electrode.55. The method of claim 36, wherein the electrochemical oxygen reductionhalf-reaction involves a transfer of at least two electrons per oxygenmolecule.
 56. An electrochemical cell comprising: a metal fuel electrodefor oxidizing a metal fuel; a low temperature ionic liquid comprisingpositive ions and negative ions; a local oxide formation promotingcompound added to the low temperature ionic liquid, the local oxideformation promoting additive dissolving in the low temperature ionicliquid to form local oxide formation promoting positive ions, whereinthe positive ions of the local oxide formation promoting compound arecoordinated with one or more negative ions forming local oxide formationpromoting positive ion-negative ion complexes; and an air electrodeconfigured to absorb and reduce oxygen, and store oxides of the metalfuel; wherein the local oxide formation promoting positive ion-negativeion complex increases the formation and storage of oxides of the metalfuel at the air electrode during discharge relative to the ionic liquidwithout the local oxide formation promoting compound.
 57. Theelectrochemical cell of claim 56, wherein the local oxide formationpromoting compound comprises a metal-containing additive, water, aprotic organic molecule containing additive, or combinations thereof.58. The electrochemical cell of claim 56, wherein the local oxideformation promoting compound comprises a metal-containing additive, andwherein the metal of the metal-containing additive is selected from thegroup consisting of Mg, Al, Mn, Ga, and Zn.
 59. The electrochemical cellof claim 56, wherein the local oxide formation promoting compoundcomprises a metal-containing additive, and wherein the metal of themetal-containing additive is different from the metal of the fuelelectrode.
 60. The electrochemical cell of claim 56, wherein theelectrochemical cell is a part of a rechargeable battery
 61. Anelectrochemical cell comprising: a fuel electrode for oxidizing a metalfuel; an ionically conductive medium comprising at least one aproticionic liquid and at least one protic ionic liquid comprising at leastone available proton per ion pair; and an air electrode configured toabsorb and reduce oxygen.
 62. The electrochemical cell of claim 61,wherein the proton improves oxygen reduction reaction thermodynamics,kinetics, or both, relative to the ionically conductive medium withoutthe at least one protic ionic liquid.
 63. The electrochemical cell ofclaim 61, wherein at least one of the aprotic ionic liquid and theprotic ionic liquid is a low temperature ionic liquid.
 64. Theelectrochemical cell of claim 61, wherein at least one of the aproticionic liquid and the protic ionic liquid is a room temperature ionicliquid (RTIL).
 65. The electrochemical cell of claim 61, wherein theaprotic ionic liquid comprises anions chloride (Cl⁻),hexafluorophosphate (PF₆ ⁻), iodide, tetrafluoroborate,bis(trifluoromethylsulfonyl)imide (C₂F₆NO₄S₂ ⁻),trifluoromethanesulfonate (CF₃O₃S⁻), derivatives and/or combinationsthereof.
 66. The electrochemical cell of claim 61, wherein the aproticionic liquid comprises cations imidazolium, sulfonium, pyrrolidinium,quaternized ammonium or phosphonium, or derivatives and/or combinationsthereof.
 67. The electrochemical cell of claim 61, wherein the aproticionic liquid comprises at least one cation that has at least onestrongly bound proton.
 68. The electrochemical cell of claim 61, whereinthe protic ionic liquid comprises anions tetrachloroaluminate,bis(trifluoromethylsulfonyl)imide, methylsulfonate, nitrate, acetate, orderivatives and/or combinations thereof.
 69. The electrochemical cell ofclaim 61, wherein the protic ionic liquid comprises cationstriethylammonium, diethylmethylammonium, dimethylethylammonium,ethylammonium, α-picolinium, pyridinium, and1,8-bis(dimethylamino)naphthalene, 2,6-di-tert-butylpyridine,derivatives of guanadines, or derivatives and/or combinations thereof.70. The electrochemical cell of claim 61, wherein the protic ionicliquid comprises at least one cation comprising at least one reversibleproton with a pKa smaller or equal to
 16. 71. The electrochemical cellof claim 61, wherein the protic ionic liquid comprises at least onecation comprising at least one reversible proton with a pKa smaller orequal to
 14. 72. The electrochemical cell of claim 61, wherein at leastone of the protic ionic liquid, the aprotic ionic liquid, and theionically conductive medium is hydrophobic.
 73. The electrochemical cellof claim 61, wherein the ratio of the concentration of the protic ionicliquid to the aprotic ionic liquid is at least about 1:100.
 74. Theelectrochemical cell of claim 61, wherein the ratio of the concentrationof the protic ionic liquid to the aprotic ionic liquid is at least about1:1.
 75. The electrochemical cell of claim 61, wherein the electricallyconductive medium further comprises a metal-containing additive, water,a protic organic molecule containing additive, or combinations thereof.76. The electrochemical cell of claim 61, wherein the air electrodecomprises polytetrafluoroethylene (PTFE).
 77. The electrochemical cellof claim 61, wherein the ionically conductive medium has a vaporpressure at or below 1 mm Hg at 20° C. above its melting point.
 78. Theelectrochemical cell of claim 61, wherein the presence of the protonproduces a shift of greater than or equal to 200 mV in the turn-onpotential for oxygen reduction versus the ionically conductive mediumwithout the at least one protic ionic liquid.
 79. The electrochemicalcell of claim 61, wherein the presence of the proton produces anincrease in the current density at the half-wave potential versus theionically conductive medium without the at least one protic ionicliquid.
 80. The electrochemical cell of claim 61, wherein the presenceof proton produces a shift of greater than or equal to about 1 V in thehalf-wave potential for oxygen reduction versus the ionic liquid withoutthe added an oxygen reduction enhancing compound.
 81. Theelectrochemical cell of claim 61, wherein the air electrode comprises acatalyst.
 82. The electrochemical cell of claim 61, wherein the airelectrode comprises a catalyst selected from the group consisting ofmanganese oxide, nickel, pyrolized cobalt, activated carbon, silver,platinum, and/or mixtures thereof.
 83. The electrochemical cell of claim61, wherein the air electrode is gas permeable.
 84. The electrochemicalcell of claim 61, wherein the air electrode comprises a barrier membraneon an outer surface of the air electrode, wherein the barrier membraneis gas permeable and liquid impermeable
 85. The electrochemical cell ofclaim 61, wherein the air electrode repels the low temperature ionicliquid.
 86. A method comprising: mixing a protic ionic liquid with anaprotic ionic liquid to create an ionically conductive medium comprisingnegative ions and positive ions, wherein at least one of the positiveions is a proton; exposing the ionically conductive medium to oxygen;and electrochemically reducing the oxygen.
 87. The method of claim 86,wherein the electrochemically reducing the oxygen occurs with improvedoxygen reduction thermodynamics, kinetics, or both, relative toelectrochemical oxygen reduction in the ionically conductive mediumwithout the protic ionic liquid.
 88. The method of claim 86, wherein theoxygen is electrochemically reduced using a catalyst.
 89. The method ofclaim 86, wherein the electrochemically reducing the oxygen occurs in anelectrochemical cell.
 90. The method of claim 86, wherein the aproticionic liquid comprises at least one cation that has at least onestrongly bound proton.
 91. The method of claim 86, wherein the proticionic liquid comprises at least one cation comprising at least onereversible proton with a pKa smaller than or equal to
 16. 92. The methodof claim 86, wherein the presence of the proton enhances reversibilityof an air cathode of the metal-air ionic liquid battery.
 93. The methodof claim 86, wherein at least one of the protic ionic liquid and aproticionic liquid is a low temperature ionic liquid.
 94. The method of claim86, wherein at least one of the protic ionic liquid and aprotic ionicliquid is a room temperature ionic liquid (RTIL).
 95. The method ofclaim 86, wherein the electrochemically reducing the oxygen occurs in ametal-air ionic liquid battery comprising a metal electrode and an airelectrode.
 96. The method of claim 86, further comprising flowing thelow temperature ionic liquid in a gap between a metal electrode and anair electrode.
 97. The method of claim 86, further comprising formingmetal-oxide by-products at a metal fuel electrode.
 98. The method ofclaim 86, further comprising storing the metal-oxide by-products at ametal electrode
 99. The method of claim 86, further comprising formingmetal-oxide by-products at an air electrode.
 100. The method of claim86, wherein the ionically conductive medium further comprises ametal-containing additive, water, a protic organic molecule containingadditive, or combinations thereof.
 101. The method of claim 86, whereinthe ionically conductive medium comprises an additive that is at leastone of triflic acid, benzonitrile: HTf, acetophenone: HTf,methanesulfonic acid, hydronium triflate, pyridazinium triflate, aceticacid, pyridinium triflate, 1,2-dimethylimidaozlium triflate,n,n-diethyl-n-methylammonium triflate, 2,2,2-trifluoroethanol,2-butyl-1,1,3,3-tetramethylguanidinium triflate, and water.
 102. Themethod of claim 86, wherein the ratio of the concentration of the proticionic liquid to the aprotic ionic liquid is at least about 1:100. 103.The method of claim 86, wherein the presence of the proton produces ashift of greater than or equal to 200 mV in the turn-on potential foroxygen reduction versus the ionically conductive medium without the atleast one protic ionic liquid.
 104. The method of claim 86, wherein thepresence of the proton produces an increase in the current density atthe half-wave potential versus the ionically conductive medium withoutthe at least one protic ionic liquid.
 105. The method of claim 86,wherein the presence of the proton produces a shift of greater than orequal to about 1 V in the half-wave potential for oxygen reductionversus the ionic liquid without the added an oxygen reduction enhancingcompound.
 106. An electrochemical cell comprising: a metal fuelelectrode for oxidizing a metal fuel; an ionically conductive mediumcomprising at least one aprotic ionic liquid and at least one proticionic liquid comprising at least one proton; an air electrode configuredto absorb and reduce oxygen and store oxides of the metal fuel, whereinthe proton increases the formation and storage of oxides of the metalfuel at the air electrode during discharge relative to the ionicallyconductive medium without the protic ionic liquid.
 107. Theelectrochemical cell of claim 106, wherein the presence of protonimproves oxygen reduction thermodynamics, kinetics, or both, relative tothe ionically conductive medium without the protic ionic liquid. 108.The electrochemical cell of claim 106, wherein the ionically conductivemedium further comprises a metal-containing additive, water, an organicmolecule, or combinations thereof.
 109. The electrochemical cell ofclaim 106, wherein the cell is a part of a rechargeable battery. 110.The electrochemical cell of claim 106, wherein both of the aprotic ionicliquid and the protic ionic liquid are a low temperature ionic liquid.111. The electrochemical cell of claim 106, wherein both of the aproticionic liquid and the protic ionic liquid a room temperature ionic liquid(RTIL).
 112. The electrochemical cell of claim 106, wherein theionically conductive medium further comprises about 10-50,000 ppm ofwater.
 113. The electrochemical cell of claim 106, wherein the ionicallyconductive medium further comprises a metal-containing additive, themetal of the additive being different from the metal of the fuelelectrode.
 114. The electrochemical cell of claim 106, wherein theoxygen reduction half-reaction involves a transfer of at least twoelectrons per oxygen molecule.
 115. The electrochemical cell of claim106, wherein the oxygen reduction half-reaction involves a transfer ofat least four electrons per oxygen molecule.
 116. The electrochemicalcell of claim 106, wherein protic ionic liquid comprises at least onecation comprising at least one reversible proton with a pKa smaller orequal to 16.